Polyimides have exhibited good thermo-oxidative stability in temperature ranges up to 300.degree. to 400.degree. C. Thus, in applications where elevated temperatures and corrosive conditions exist, polyimides have been suggested for extensive application. Early polyimides exhibited a low percentage of elongation to break and utility was limited.
U.S. Pat. No. 3,812,082 teaches a compliant or highly flexible polyimide produced from a condensation reaction of a dianhydride and a diamine or diisocyanate. These polyimides exhibit an elongation to break of up to 300 percent, and a set at break of 25 percent or less. Thus, where a high performance sealing material is required and it can be cast, such polyimides are very suitable.
A significant advance in a practical route to flexible polyimides is taught in U.S. Pat. No. 3,951,902. In this addition-type poly(Diels-Alder) approach, a melt polymerization reaction yields polymers acceptable for use at temperatures up to 288.degree. C. The polyimides processed by this melt process possess properties equivalent to the solution route employed in U.S. Pat. No. 3,812,082. Thus, a melt process is available to provide flexible polyimides which possess a high elongation to break and can be melt extruded or cast into a variety of products including seals, sealants, adhesives and coatings. The availability of melt and hot melt product fabrication processes means substantial cost reduction and significant lessening of human hazards and environmental pollution over polyimides produced and processed by conventional solution methods.
U.S. Pat. No. 3,652,511 teaches a water-dispersible polyimide coating formed by reacting maleic anhydride with an aliphatic diamine in solution. The polyimide product can be formed into a hard and solvent resistant film. Elastomeric or high recoverable elongation characteristics are not inherent to the solution-produced polyimide itself and are achieved only by a copolymer reaction such as reacting a sulfhydryl-terminated polymer, such as nitrile rubber, with the maleic unsaturation in the bismaleimide or the acidic side chain. This approach significantly limits or excludes their utility in terms of applications where use in seal, sealant adhesive and coating use is required at temperatures of 120.degree. C. or greater. Also, such resins have pendant carboxyl groups which result in water solubility. This diminishes their utility for general engineering applications due to possible unfavorable side reactions in high temperature use environments.
Similarly, U.S. Pat. No. 2,818,405 teaches elastomeric polyimides formed by the equal molar reaction of bismaleimides and free organic diamines. The organic diamines employed in this invention are hydrocarbon or halogenated hydrocarbon segments which restrict temperature performance to 93.degree. C. or below. Also, the technology disclosed necessitates the use of organic tri- or tetraamines to accomplish cure of the linear imide resin initially produced. Use of the technology described therein severely limits achievement of a broad range of polymer mechanical properties because of the requirements to use 1:1 molar reactant stoichiometry.
In U.S. Pat. No. 4,116,937 assigned to the same assignee of this invention and incorporated herein by reference there is described precursors produced by the Michael addition reaction of an aromatic diamine with an aromatic maleimide and a maleimide terminated polyaliphatic ether by a melt process and a cross-linked final product produced by a cure reaction. The addition reaction which forms the maleimide terminated precursor occurs in the melt at temperatures ranging from 100.degree. to 150.degree. C. Subsequently, when the temperature is raised to between 160.degree. C. and 200.degree. C., the precursor cures by a crosslinking reaction requiring no additional additives or catalysts.
There is a need in the art for polyimide sealants which can be readily cured at low temperatures such as from room temperature to about 120.degree. F. This cure should be effected by the use of readily available ingredients and should produce a sealant with improved properties over conventional polysulfide sealants.
It has been determined that an aliphatic bismaleimide alone or modified with an aromatic bismaleimide and/or an aromatic diamine can be made to cure in the presence of a crosslinking agent having at least two vinyl groups and an active free radical catalyst at a temperature of 65.degree. C. or less. A trismaleimide and an acid catalyst can be used in place of the crosslinking agent and free radical catalyst. Careful control, however, is required over proportions of reactants to achieve a product of acceptable properties.
Since the ingredients which form a curable system are often formulated just before application to a substrate and often by unskilled workers, it would be desirable to have a low temperature curable system which is less sensitive to deviations in proportions of ingredients. It is also desirable to provide a system where equal parts by weight or volume can be combined to achieve cure such that non-quantitative judgements can be made in metering reactants with assurance that a useful product will be formed when cure is perfected.